import java.util.*;
public class Test {
static List list;
static Set set;
public static void main(String[] args){
Student stu1 = new Student("001","zhang","100");
Student stu2 = new Student("002","zhao","200");
Student stu3 = new Student("003","li","300");
Teacher tea1 = new Teacher("004","wang","400");
Teacher tea2 = new Teacher("005","yang","500");
List list = new ArrayList();
list.add(stu1);
list.add(stu2);
list.add(stu3);
list.add(tea1);
list.add(tea2);
Test.show();
list.clear();
Test.show();

}
private static void show() {
for(int i = 0; i<list.size(); i++){
Person p = (Person)list.get(i);
p.eat();
}
}
}
出现了空指针异常，这是为什么？